To debate or not to debate? and When to debate? and How to debate?

I think we should debate following the Lord Buddha's example in MN 35:

Suppose, Aggivessana, there was a person in need of heartwood. Wandering in search of heartwood, they’d take a sharp axe and enter a forest. There they’d see a big banana tree, straight and young and flawlessly grown. They’d cut it down at the base, cut off the top, and unroll the coiled sheaths. But they wouldn’t even find sapwood, much less heartwood. In the same way, when engaged, pressed, and examined by me on your own doctrine, you turn out to be void, hollow, and mistaken. But it was you who stated before the assembly of Vesālī: ‘If I was to take them on in debate, I don’t see any ascetic or brahmin—leader of an order or a community, or the teacher of a community, even one who claims to be a perfected one, a fully awakened Buddha—who would not shake and rock and tremble, sweating from the armpits. Even if I took on an insentient post in debate, it would shake and rock and tremble. How much more than a human being!’ But sweat is pouring from your forehead; it’s soaked through your robe and drips on the ground. While I now have no sweat on my body.” So the Buddha revealed his golden body to the assembly. When this was said, Saccaka sat silent, embarrassed, shoulders drooping, downcast, depressed, with nothing to say.

Knowing this, the Licchavi Dummukha said to the Buddha: “A simile strikes me, Blessed One.”

"Then speak as you feel inspired,” said the Buddha.

“Sir, suppose there was a lotus pond not far from a town or village, and a crab lived there. Then several boys or girls would leave the town or village and go to the pond, where they’d pull out the crab and put it on dry land. Whenever that crab extended a claw, those boys or girls would snap, crack, and break it off with a stick or a stone. And when that crab’s claws had all been snapped, cracked, and broken off it wouldn’t be able to return down into that lotus pond. In the same way, sir, the Buddha has snapped, cracked, and broken off all Saccaka’s tricks, dodges, and evasions. Now he can’t get near the Buddha again looking for a debate.”

I don't think we should be afraid of debating with people who have different views to our own. The Buddha himself was very happy to do just that. In the Assalayana Sutta the Buddha has a full and frank exchange of opinions with a young Brahmin. Just to give a flavour of the discourse

What do you think, Assalayana? Is it only a noble warrior who — taking life, stealing, engaging in sexual misconduct, telling lies, speaking divisively, speaking harshly, engaging in idle chatter, greedy, bearing thoughts of ill will, and holding wrong views — on the break-up of the body, after death, reappears in the plane of deprivation, the bad destination, the lower realms, in hell, and not a brahman? Is it only a merchant...? Is it only a worker [...]

And again in the Upali sutta the Buddha debates with a Jain about points of doctrine. Again another flavour of the robust debate

Then Dighatapasssi said to the Blessed One: ‘Friend Gotama, how many punishments do you declare for doing demerit and its perpetration?’ ‘Friend, Tapassi, it is not the practise of the Thus Gone One to say punishment, it is the practise to say,action.’‘Friend Gotama, how many actions do you declare for doing demerit and its perpetration?’. ‘Tapassi, I declare three actions for doing demerit and its perpetration. They are bodily, verbal and mental actions.

So I would say that debate is squarely in the Buddhist tradition, certainly as understood by the Theravadans. However as other posters have said this needs to be done with the context of a metta-ful response.

My answer is not bas3d on scriptural references but on experiences I had both in scientific community as well as religious groups.

1. The very first point is to make sure you are not doing it for Ego gratification. The purpose of 'debate' per se is to win over other. However this should not be the case. You should try in utmost polite language to get your poibt across. If the other party still hold on to their opinion you should end it up as just that-- 'their opinion'.

2. Make sure the dialogue exchanged contributes to exchange of knowledge and you both together end up with broadened line of thinking. Dont use Ad Hominem arguments.

3. If the debate gets centered around things which cannot be proved or confirmed no matter what...for e.g. you enter into debate 'wheater there really is a heavenly or hellish realm as told in scriptures or its just a metaphor'. Now no amount of intellectual and logical thought exchange is going to settle that. In such cases dont go into debates. Its a waste of time and mental energy and moreover it leads the sangha nowhere.

Debate in the worldly sense has no much purpose in buddhism. What is there to win in a debate in Buddhism? If I debate you and win you over, what does it mean to me?

I would debate only if there is something for me to learn out of it. That would be debating to loose. Can you call this debating?

Another thing would be digging deep on a teaching with another person using debating as a tool. This is two people trying to understand something deeper But that's not really debating ether.

To my view only situation it may work debating in the worldly way is if you know the other person is misguided or being misguided and you know for sure that you can help them see the truth without confusing them any further. This is primarily what Buddha debated for.

In the suttas, there are many helpful instructions for conduct discussions and exposing the Dhamma. Here are some excerpts from Aranavibhanga Sutta (MN 139, Nanamoli translation):

So, bhikkhus, how does there come to be over-rating and under-rating and failure to speak only Dhamma?

When a man says “All those engaged in such pursuit of enjoyment, which pleasure is linked to sensual desires, low, vulgar, coarse, ignoble and connected with harm, beset by pain, by vexation, by despair and by fever, and they have entered upon the wrong way,” he thus under-rates some.

When a man says, “All those not engaged in such pursuit of enjoyment which pleasure is linked to sensual desires, low, vulgar, coarse, ignoble and connected with harm, are without pain, without vexation without despair, and without fever, and they have entered upon the right way,” he thus over-rates some.

[...]

This is how there comes to be overrating and underrating and failure to speak only Dhamma.

And bhikkhus, how does there come to be neither overrating nor underrating but speaking only Dhamma?

When a man does not say, “All those engaged in such pursuit of self-mortification, painful, ignoble, and connected with harm, are beset by pain, by vexation, by despair and by fever, and they have entered upon the wrong way,” and says instead “It is the “being engaged” that is a state beset by pain, by vexation, by despair and by fever, and it is the wrong way,” then he speaks only Dhamma.

and:

He should not utter covert speech, and he should not utter overt, harsh speech.” So it was said. And with reference to what was this said?

Here, bhikkhus, when a man knows covert speech to be untrue, incorrect and harmful, he should on no account utter it. When he knows covert speech to be true, correct and harmful, he should try not to utter it. But when he knows covert speech to be true, correct and beneficial, he may utter it, knowing the time to do so.

Here, bhikkhus, when a man knows overt, sharp speech to be untrue, incorrect and harmful, he should on no account utter it. When he knows overt, sharp speech to be true, correct and harmful, he should try not to utter it. But when be knows overt, sharp speech to be true, correct and beneficial, he may utter it, knowing the time to do so.

Also, in Abhayarājakumāra (MN 58, Nanamoli/Bodhi), the Buddha exposes his own criteria for uttering speeches:

So too, prince, such speech as the Tathagata knows to be untrue, incorrect and unbeneficial, and which is also unwelcome and disagreeable to others: such speech the Tathagata does no utter.

Such speech as the Tathagata knows to be true and correct but unbeneficial, and which is also unwelcome and disagreeable to others: such speech the Tathagata does not utter.

Such speech as the Tathagata knows to be true, correct and beneficial, but which is unwelcome and disagreeable to others: the Tathagata knows the time to use such speech.

Such speech as the Tathagata knows to be true and correct but unbeneficial, and which is welcome and agreeable to others: such speech the Tathagata does not utter.

Such speech as the Tathagata knows to be true, correct, and beneficial and which is welcome and agreeable to others: the Tathagata knows the time to use such speech. Why is that? Because the Tathagata has compassion for beings.

Finally, in Kosambya (MN 48) the Buddha exposes principles of cordiality which provides a broader foundation for the specific orientations above.

from Kathāvatthu Sutta:

Monks, it’s through his way of participating in a discussion that a person can be known as fit to talk with or unfit to talk with. If a person, when asked a question, wanders from one thing to another, pulls the discussion off the topic, shows anger & aversion and sulks, then—that being the case—he is a person unfit to talk with. But if a person, when asked a question, doesn’t wander from one thing to another, doesn’t pull the discussion off the topic, doesn’t show anger or aversion or sulk, then—that being the case—he is a person fit to talk with.

Monks, it’s through his way of participating in a discussion that a person can be known as fit to talk with or unfit to talk with. If a person, when asked a question, puts down (the questioner), crushes him, ridicules him, grasps at his little mistakes, then—that being the case—he is a person unfit to talk with. But if a person, when asked a question, doesn’t put down (the questioner), doesn’t crush him, doesn’t ridicule him, doesn’t grasp at his little mistakes, then—that being the case—he is a person fit to talk with.

...

For that’s the purpose of discussion, that’s the purpose of counsel, that’s the purpose of drawing near, that’s the purpose of lending ear: i.e., the liberation of the mind through lack of clinging/sustenance.

Those who discuss
when angered, dogmatic, arrogant,
following what’s not the noble ones’ way,
seeking to expose each other’s faults,
delight in each other’s misspoken word,
slip, stumble, defeat.
Noble ones
don’t speak in that way.

If wise people, knowing the right time,
want to speak,
then, words connected with justice,
following the ways of the noble ones:
That’s what the enlightened ones speak,
without anger or arrogance,
with a mind not boiling over,
without vehemence, without spite.
Without envy
they speak from right knowledge.
They would delight in what’s well-said
and not disparage what’s not.
They don’t study to find fault,
don’t grasp at little mistakes.
don’t put down, don’t crush,
don’t speak random words.

From AN 5.165:

All those who ask questions of another do so from any one of five motivations.
Which five?

One asks a question of another through stupidity & bewilderment. One asks a question of another through evil desires & overwhelmed with greed. One asks a question of another through contempt. One asks a question of another when desiring knowledge. Or one asks a question with this thought,1 'If, when asked, he answers correctly, well & good. If not, then I will answer correctly

Sn 4.8 Pasura Sutta:

"Only here is there purity"
— that's what they say —
"No other doctrines are pure"
— so they say.
Insisting that what they depend on is good,
they are deeply entrenched in their personal truths.

Seeking controversy, they plunge into an assembly,
regarding one another as fools.
Relying on others' authority,
they speak in debate.
Desiring praise, they claim to be skilled.

Engaged in disputes in the midst of the assembly,
— anxious, desiring praise —
the one defeated is
chagrined.
Shaken with criticism, he seeks for an opening.

He whose doctrine is [judged as] demolished,
defeated, by those judging the issue:
He laments, he grieves — the inferior exponent.
"He beat me," he mourns.

These disputes have arisen among contemplatives.
In them are elation,
dejection.
Seeing this, one should abstain from disputes,
for they have no other goal
than the gaining of praise.

He who is praised there
for expounding his doctrine
in the midst of the assembly,
laughs on that account & grows haughty,
attaining his heart's desire.

That haughtiness will be his grounds for vexation,
for he'll speak in pride & conceit.
Seeing this, one should abstain from debates.
No purity is attained by them, say the skilled.

Like a strong man nourished on royal food,
you go about, roaring, searching out an opponent.
Wherever the battle is,
go there, strong man.
As before, there's none here.

Those who dispute, taking hold of a view,
saying, "This, and this only, is true,"
those you can talk to.
Here there is nothing —
no confrontation
at the birth of disputes.

Among those who live above confrontation
not pitting view against view,
whom would you gain as opponent, Pasura,
among those here
who are grasping no more?

So here you come,
conjecturing,
your mind conjuring
viewpoints.
You're paired off with a pure one
and so cannot proceed.

Most people approach debate as if it was a sword fight. They debate to win, as if losing meant death. They do not make any effort to understand the other person's perspective!

They keep on fighting until they see an opening in the opponent's defense. As soon as they see a little inaccuracy or a logical error, they happily grasp on that -- even if it does not change the main idea the other person is trying to convey.

When the other person gets confused for a moment and says something obviously wrong, they happily make fun of him and keep pushing until the opponent is completely lost.

This kind of people firmly hold their ground, their confidence coming from their limited perspective and narrow view. Having won another debate with aggression and cunning, they are proud of themselves.

But if they see they cannot win, they get very envious of the winner, they get nervous, start changing topics, lie, resort to personal attacks, get angry, and finally go down to rudeness and yelling.

Instead, the noble students do not debate, they have conversations. They discuss things. When they thus sincerely talk they try to see each other's perspective. They understand that each (truthful) perspective is valid from its own side. So they try to see the value of the other perspective, and appreciate the factors and conclusions of the other side. Then, by combining the two perspectives, they come out with a more complete, more thorough understanding of the original problem. Each side lets go of its attachments and delusions as it accepts the other side's point of view, and integrates them together.

Also, see MN18 for an in-depth analysis of the psychology of reification underlying debates and arguments.